Advancements in semiconductor devices and packaging technology has been a principal agent of change in printed wiring board (PWB) technology. To extract maximum system performance, the characteristic impedance of transmission lines on a PWB must match that of the semiconductor devices mounted on it in order to prevent signal reflection or ringing which can cause distortion of digital signals between devices on a PWB.
Impedance is a property of all conductors and refers to the resistance to the flow of electrical current in a conductor pair when voltage is applied across the pair. Characteristic impedance is a material dependent parameter that is indicative of the high frequency behavior of a PWB. Characteristic impedance has an expanding role in modern electronics due to increasing logic and system speeds. Characteristic impedance of a transmission line is primarily measured using time domain reflectometry (TDR) techniques.
When using TDR techniques to measure characteristic impedance the units of measure are derived and not directly measured. The accuracy of such measurements can not be directly traced to NIST (the National Institute of Standards and Technology). Therefore, accuracy and repeatability of characteristic impedance measurements performed by using TDR techniques has been the subject of much research. TDR measurements often vary greatly depending on how the tests are performed.
In order to achieve repeatable and accurate measurements using TDR techniques, the electromagnetic losses of the line under measurement and the respective interconnections to the TDR instrumentation must be compensated for. Prior art methods don't compensate completely for these losses thereby, producing inaccurate results. One prior art method attempts to compensate for line loss by making DC resistance measurements. However, such an approach does not compensate for losses that result from the high frequency content of the signal used for TDR measurements. Another prior art method is described in U.S. Pat. No. 5,063,353.